A. Field of the Invention
The present invention relates to a high-grade non-oriented silicon steel sheet, and more particularly, to a non-oriented silicon steel sheet excellent in magnetic properties which contains 1.5-3.5% Si, not more than 0.015% C, and not more than 0.08% acid-soluble Al (referred to as "sol Al" hereinafter), and which has B added thereto to obtain a balanced ratio of B to N and further contains a rare earth element.
B. Description of the Prior Art
Among silicon steel sheets constituting soft magnetic materials, there are the following kinds: a grain-oriented silicon steel sheet composed of a recrystallized collective texture crystallographically expressed as (110) [001] having its (110) plane on the rolling plane and its [001] orientation in the rolling direction; and a non-oriented silicon steel with negligible orientation. These materials are used for the iron cores of electrical devices chosen for specified applications in accordance with their properties (magnetic, mechanical etc.) and their production cost.
For instance, grain-oriented silicon steel sheet is generally used for transformers and pole transformers of large capacity regardless of its high cost because it has excellent magnetic properties. Namely, it is very easily magnetizable in the rolling direction, that is in the [001] direction, has a very low value of watt loss, and has very high permeability.
On the other hand, however, non-oriented silicon steel sheet is usually categorized into the following two kinds: low-grade non-oriented silicon steel sheet having a Si content between near zero and 1.5%, and high-grade non-oriented silicon steel sheet containing more than 1.5% Si. As these materials have an orientation so small as to be negligible and are low in cost, they are widely used for small-sized electric motors, medium-sized transformers, and medium-to-large-sized rotating machines.
A key property required of non-oriented silicon steel sheet is a low watt loss value. Watt loss consists chiefly of hysteresis loss and eddy current loss. In non-oriented silicon steel sheet, hysteresis loss is most effectively reduced by making the crystal grain size large and eddy current loss can be reduced by increasing the specific resistance of the steel, in other words by adding an alloying element such as Si or the like.
One means for enlarging the crystal grain size is by high-temperature annealing carried out over an extended period of time. This is, however, economically disadvantageous. Moreover, since continuous annealing is usually applied, long annealing times are difficult to obtain.
Inhibited growth of crystal grain size is known to be closely related to dispersed inclusions and precipitates. The size of the inclusions and precipitates and the state of their dispersion are very important. It is considered in general that the presence of many fine inclusions and precipitates is not beneficial for the growth of the crystal grains. As substances contained in dispersed phase in non-oriented silicon steel sheet, there can be mentioned various oxides, such as Al.sub.2 O.sub.3, SiO.sub.2 and the like, and various sulphides and nitrides, such as MnS and AlN.
As for the oxides, it is only necessary to collect and float them upwardly as deoxidized products in the steel making step and to prevent oxidizing at the time of casting the steel metal into ingot molds. Owing to recent progress in steel making techniques, inclusion of oxides can be easily prevented.
A widely used method for contending with sulphides is to thoroughly carry out desulphurizing in the steel making step while, at the same time, adding a suitable amount of Mn or the like, and then maintaining the heating temperature of the slab prior to the hot rolling step so low that the MnS can be converted to a substance harmless to the growth of the crystal grains. Because of the use of this method, sulphides do not pose a problem.
To hold down the inclusion of nitrides, it is best to keep the nitrogen content as low as possible in the steel making step. It is, however, very difficult to consistently reduce the nitrogen content in silicon steel to less than 10 ppm, and about 30 ppm of nitrogen is usually contained.
The combination of N with Al used for deoxidizing or with Al contained in a silicon alloy results in the formation of AlN. As an Al content in the range of 0.005-0.15% is injurious to the growth of the crystal grains, it has been usual in the production of low-grade non-oriented silicon steel sheet containing less than about 1.0-1.5% Si to reduce the Al used for deoxidizing as much as possible, preferably to less than 0.005%. Besides, in high-grade non-oriented silicon steel sheet containing more than about 1.0-1.5% Si, the Al content is controlled to be more than 0.15% and AlN is precipitated in a form harmless to the growth of the crystal grains when the steel slab is heated.
However, in the former case, there arise such troubles as insufficient deoxidation in the steel making step or the occurrence of defects on the surface of the steel sheet. In the latter case, production cost inevitably rises due to the consumption of a large quantity of expensive Al.
As an effective method for stabilizing deoxidation in the steel making process while avoiding the rise in cost caused by the use of large amounts of Al, the inventors previously developed an inexpensive method for manufacturing non-oriented silicon steel sheet having an excellent watt loss value by controlling the ratio of B to N to fall within the range of 0.5-2.50 by the addition of B. It has been found, however, that when steel sheet having a very low C content (0.015% or less) is produced by the method, the product is not entirely uniform in its magnetic properties.